As systems, such as the multimedia entertainment, communications, process control, and diagnostic systems utilized by the automotive and aerospace industries, become more complex, a need arises for additional devices to communicate, either with each other or with a central controller or the like. Historically, these systems included dedicated wiring extending between the various devices in order to support communications therebetween. As systems have become more integrated and the communications requirements have been increased, the amount of dedicated wiring that would be required has become excessively large, both in terms of the space required for the wiring and the cost of the wiring and the attendant installation. Moreover, as the amount of dedicated wiring increases, the overall complexity of the system also generally increases, as well as the likelihood that some portion of the wiring might be damaged or broken during or following installation.
As such, network systems have been developed to provide a common communications path between a plurality of network devices. In automotive and aerospace applications, for example, a network system can be utilized to monitor various components and to collect diagnostic and status information. In this regard, diagnostic and status information relating to the strain, acceleration, pressure and/or temperature to which the various components are subjected may be collected and analyzed.
By way of further example, a network bus architecture is currently being developed to support communications and the delivery of multimedia information to the occupants of a vehicle such as an automobile, minivan, sports utility vehicle, aircraft, boat or the like, or for use in process control applications that could be applied to a system such as a vehicle. Advantageously, this network bus would transport the audio signals, including streaming audio signals, produced by one or more of a radio, a cassette tape player, a compact disc player or the like to selected speakers or headphone jacks throughout the vehicle. Similarly, the network bus may support voice and data communications with a cellular telephone carried by an occupant of the vehicle, as well as communications with a laptop computer, a handheld computing device or the like. In addition, the network bus may transmit video signals, including streaming video signals, from a television receiver, a videocassette recorder or other video source to one or more video monitors. The system may also support process control applications such as automotive engine and transmission control, drive-by-wire, brake-by-wire, passive restraint, and/or collision avoidance systems that require highly reliable fault tolerant systems. Each of these systems could include a multitude of sensors and actuators that can be controlled using the network architecture under development. Further, the network bus system may transport information related to diagnostic performance of the vehicle.
In addition to the variety of devices that are connected to a network bus, one or more controllers are also generally connected to the network bus for sending commands to the various remote devices and receiving data from the remote devices. Among other things, these commands specify the manner in which the various devices are to function including the manner in which the various devices are to transmit information over the network bus.
Traditionally, networks of the type described above have transmitted data in analog format. Unfortunately, analog signals are susceptible to noise introduced into the signals during data transmission. Given that many of the transmitted signals have a low amplitude to start with, this noise can corrupt the signal and decrease the signal to noise ratio to levels that cause loss of resolution in the signal. Further, as many of these network devices are scattered some distance from the controller, the electrical lines connecting the network devices to the controller may be sufficiently long to cause signal degradation due to DC resistance in the wiring.
In light of these shortcomings, it would be advantageous to utilize digital networks. But, many conventional digital networks suffer from a variety of problems themselves. For example, many existing digital networks operate according to complicated protocols which require each network device to have a relatively high level processor, thereby increasing the cost of the network devices. Complicated protocols also introduce overhead into the messages on the bus that is not necessary for data acquisition and control. This overhead can severely limit the number of data samples that can be transmitted on the bus. These networks also have other problems. For example, they generally do not support both acquisition and control, and they typically only support networks that extend over relatively short lengths. Further, these networks typically have bulky network device interfaces, slow network data rates and/or a low network device count. Additionally, many computer systems that include digital networks do not operate in a time-deterministic manner. As such, these computer systems generally lack the capability to schedule a trigger on event command to the network components that repeats or is interpreted and executed with any precision.
Further, some conventional network systems operate in a master/slave configuration, where the bus controller controls all communication on the network bus. In these network systems, the bus controller uses an operation schedule that includes commands related to either one or a group of network devices and the timing for communicating with the network devices. As such, the bus controller mandates all or substantially all timing for communications.
Although placing the bus controller in control of all communications on the network bus is advantageous as it eliminates communication conflicts on the network bus, there may be some drawbacks to this configuration. Specifically, in some instances, it is desirable to move data directly from one or more network devices to other network devices without interdiction from any network controller, such as in the case of moving streaming audio signals.
To allow for peer-to-peer communication among network devices, some conventional network systems include dedicated wiring between various network devices for providing direct access between the network devices. As stated above, however, an important advantage of network systems is the replacement of dedicated wiring with a common network bus. The requirement of the additional dedicated wiring by the above-described network system is not consistent with this goal and suffers from problems, such as added cost of the wiring and the attendant installation, as well as the likelihood that some portion of the wiring might be damaged or broken during or following installation.
As an alternative to the use of dedicated wiring, other network systems require communications between network devices to proceed through the network controller. This alternative, however, is also undesirable. Because a constant flow of information is often transmitted by one network device and received by other network devices, the timing of the transfer of information is critical to the success of the transfer. Because transferring information through the network controller adds an extra element through which the information must travel, it unnecessarily adds time to the overall information transfer.